Excessive heat during draawing of high carbon wire causes high wire temperature that reduces ductility. When wire temperatures remain above 375.degree. F. for more than several seconds, strain ageing embrittles the wire, increasing its tensile strength but reducing torsional and other ductility. Therefore, wire temperature should be kept as low as possible.
The following factors affect the generation of drawing heat:
drawing speed;
pass area reduction; and
friction in the die.
Reducing the drawing speed reduces drawing heat, and therefore reduces wire temperature, but is uneconomical because of the loss of productivity.
Decreasing the area of reduction at each draft is also uneconomical since more drafts are required which necessitates a larger machine.
Lubricants are used to reduce die friction. However, high temperatures cause lubricant breakdown which can lead to poor wire quality (as a result of surface scratching), wire breakage, and reduced die life.
Historically, the heat introduced into the wire has been removed by water-cooled capstans assisted by forced air. This method of cooling has reached its limit of effectiveness and the industry has progressed to directly contacting the wire with coolant. Direct wire cooling has enabled operators either to increase drawing speeds while maintaining wire quality, or to produce superior quality wire at existing speeds.
Canadian Pat. No. 1,006,119, issued Mar. 1, 1977 to Kobe Steel, Ltd., attempted to increase drawing speeds without increasing drawing heat by providing a water-cooled drawing die. Water flows around the die, and then through a tube attached to the die. The water in the tube area directly contacts the wire. This apparatus allows increased speeds without affecting wire quality but has the following disadvantages:
the wire must be provided with a long tapered point during threading, since the apparatus is not removeable for string-up;
existing die holders and soap boxes must be modified;
if the wire breaks in the die, water will get into the soap box causing lubricant break-down;
residual cooling water is removed from the wire surface by an air wipe which can be noisy; and
cooling water chills the hot die and can cause die cracking.
Canadian Pat. No. 1,044,181, issued Dec. 12, 1978 to G. K. N. Somerset Wire Limited, attempts to maintain high carbon wire at low temperature (to control strain ageing) by spraying high pressure water directly onto the wire as it passes from the drawing die to the capstan. The apparatus described has several disadvantages:
the water spray is not contained and as a result humidity is increased which can lead to product rusting;
the high pressure air used to wipe the sprayed product creates an unacceptably high level of noise; and
the cooling device rests, or rides, on the wire being processed, causing wear.
Accordingly, a primary aspect of this invention is to develop a satisfactory and improved technique for using water-spraying to cool drawn wire.
More particularly, it is known that temperatures out of a typical die are about 450.degree. F. for prestressed concrete (PC) strand wire drawn at high speeds. The wire should be cooled to below 350.degree. F. as quickly as possible, and it has been found that spraying water on a single wire before it reaches the block is one way to accomplish such cooling.
Generally speaking the criteria for satisfactory performance of a cooling device as envisioned herein are as follows:
fast cooling (i.e. 450.degree. F. to 350.degree. F. in 0.5 sec);
device must contain all steam, spray, and water (i.e. no leakage);
wire must be dry when it leaves the device;
device must fit in the available space between the die box and block;
must be movable to allow normal string-up;
must not generate excessive noise; and
must be suitable for various wire sizes.
In order to obtain fast cooling the method and apparatus of this invention, as applied to the cooling of drawn wire, proposes to spray the wire with water.
In regard to containment, it is important that all steam generated, all overspray and water, must be contained in the cooling device. Leakage to the atmosphere should be minimized because if the air around the soap boxes becomes too moist, the soap lubricant absorbs water and will not perform properly. Containment is achieved, in accordance with the present method and apparatus, by drawing a vacuum on the cooling device.
With respect to the drying of the wire, it is proposed that all moisture be removed from the surface of the wire by air wiping. The exit from the spray box is sized such that when the proper air flow is drawn by the vacuum system, a high velocity air wipe is created. Any residual dampness on the wire will evaporate quickly since the wire temperature is well above the boiling point of water.
The cooling device of this invention can be made compact and rugged, in order to fit the space between the die box and the block. The wire does not have to pass through the geometric center of the device for water sprays to cool it effectively. The length of the device can be adjusted to fit the available space. If the device must be shorter than optimum, a greater water flow can be used to increase the cooling. The cooling device is preferably loosely held in place so that it can "float" with the moving wire and be aligned by it.
In order to allow the cooling device to be removed to allow string-up, the device is split longitudinally and hinged on one side so that it can be opened and placed around a wire already strung up. Inserts at both ends are also split. The device is connected by flexible hoses to the water supply and the vacuum pump.
With respect to noise generation, it is known that high pressure air wipes usually generate noise at levels requiring ear protection, because the air is exhausted to the atmosphere. However with the present device, the cooling noise cannot be heard above the ambient noise because a low pressure vacuum is used, and because the air wipe exhausts into the device.
By utilizing hardened inserts at the ends of the cooling device, constituting wear guides and air wipe orifices, the device can be adapted to various wire sizes. The outside diameter of the inserts is standardized so that the inserts are interchangeable. The inside diameters are sized for various wire gauges.
It should be pointed out that the principle of this invention could be utilized to carry out operations other than cooling. For example, the treatment of a drawn wire or extruded product with a liquid could be carried out separately or at the same time as the cooling.
Accordingly, this invention provides a method for treating wire, the method comprising several steps:
First, a housing is provided to define a chamber having an inlet for the wire and an outlet for the wire. Both the inlet and the outlet are larger in area than the maximum cross-sectional area of the product. The wire is passed through the chamber from the inlet to the outlet, while liquid is sprayed against the wire. Simultaneously, suction means is caused to withdraw air and liquid from the chamber at a rate which results in an air-wiping effect on the wire due to the entry of ambient air through the inlet and outlet of the chamber.
Additionally, this invention provides an apparatus for treating wire, the apparatus including a housing defining a chamber having an inlet for the wire and an outlet for the product, both the inlet and the outlet being larger in area than the maximum cross-sectional area of the wire. Nozzle means are provided within the chamber for spraying liquid against the wire. Additionally, supply means are provided for supplying liquid under pressure to the nozzle means, and suction means communicate with the chamber for withdrawing air and liquid from the chamber at a rate which results in an air-wiping effect on the wire due to the ambient air entering the chamber through the inlet and outlet.